Propulsion

How are rockets designed?

Rocket designers want the rocket to do the best job possible for its mission.
The performance of rocket engines can be measured in several ways, and the designer
must decide which kinds of performance he or she would like the rocket to emphasize.

Some important questions for rocket engine designers are below:

How powerful is the rocket; how much thrust can the motor produce? This
is important because the rocket must be powerful enough to counteract Earth's
gravity, and get its payload (the stuff that the spacecraft is carrying) into
orbit, or even out of orbit!

What is the power-to-weight ratio? This is important because the
heavier the engine is, the harder it will be to get the spacecraft into space.
However bigger (heavier) engines can be much stronger than small light ones.
If you make a light enough spacecraft, it may not have enough thrust. So if
a rocket is heavy, it must be strong, and if it is weak, it should be light.

What is the speed of the exhaust gases? The faster the exhaust gasses
stream out, the more thrust, and thus the faster the ship goes forward.

How long can it run? The rocket has to get its payload to its destination
against gravity. If the rocket runs out of oomph too quickly, the rocket may
fall back to Earth or put its payload into a completely wrong orbit.

No rocket design or kind of propellant will give the best answer to all of
these questions. There are always tradeoffs; depending on what the satellite
needs different kinds of rockets are chosen. The designer must choose which
qualities are most important to his or her design and this changes depending
on the rocket's intended purpose. Sometimes a single mission will have more
than one propulsion system for different kinds of propulsion.